1. Field of the Invention
The present invention relates to thermal printers and, more particularly, to techniques for controlling thermal print heads.
2. Related Art
Thermal printers typically contain a linear array of heating elements (also referred to herein as “print head elements”) that print on an output medium by, for example, transferring pigment or dye from a donor sheet to the output medium or by activating a color-forming chemistry in the output medium. The output medium is typically a porous receiver receptive to the transferred pigment, or a paper coated with the color-forming chemistry. Each of the print head elements, when activated, forms color on the medium passing underneath the print head element, creating a spot having a particular density. Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
A thermal print head element is activated by providing it with energy. Providing energy to the print head element increases the temperature of the print head element, causing either the transfer of pigment to the output medium or the formation of color in the receiver. The density of the output produced by the print head element in this manner is a function of the amount of energy provided to the print head element. The amount of energy provided to the print head element may be varied by, for example, varying the amount of power to the print head element within a particular time interval or by providing power to the print head element for a longer time interval.
In conventional thermal printers, the time during which a digital image is printed is divided into fixed time intervals referred to herein as “print head cycles.” Typically, a single row of pixels (or portions thereof) in the digital image is printed during a single print head cycle. Each print head element is typically responsible for printing pixels (or sub-pixels) in a particular column of the digital image. During each print head cycle, an amount of energy is delivered to each print head element that is calculated to raise the temperature of the print head element to a level that will cause the print head element to produce output having the desired density. Varying amounts of energy may be provided to different print head elements based on the varying desired densities to be produced by the print head elements.
One problem with conventional thermal printers results from the fact that their print head elements retain heat after the conclusion of each print head cycle. This retention of heat can be problematic because, in some thermal printers, the amount of energy that is delivered to a particular print head element during a particular print head cycle is typically calculated based on an assumption that the print head element's temperature at the beginning of the print head cycle is a known fixed temperature. Since, in reality, the temperature of the print head element at the beginning of a print head cycle depends on (among other things) the amount of energy delivered to the print head element during previous print head cycles, the actual temperature achieved by the print head element during a print head cycle may differ from the desired temperature, thereby resulting in a higher or lower output density than is desired. Further complications are similarly caused by the fact that the current temperature of a particular print head element is influenced not only by its own previous temperatures—referred to herein as its “thermal history”—but by the ambient (room) temperature, the thermal histories of other print head elements in the print head, and the temperature of the output medium (film/media) and other thermal printer elements, such as the platen roller and the preheat contact with the thermal heat sink of the Thermal Print Head (TPH).
Various techniques have been applied to counterbalance these undesirable effects of the thermal history of a thermal print head. Such techniques are referred to generally as “thermal history control.” Examples of such techniques are disclosed in the above-referenced patent application entitled “Thermal Response Correction System.”
Different numbers and combinations of thermal print head elements may be active at different times when printing a digital image, depending on the intensities of the pixels in the digital image. As a result of the circuitry that is typically used to provide power to the print head elements in a thermal printer, spots that are printed by a large number of contemporaneously active print head elements appear lighter than spots that are printed by a small number of contemporaneously active print head elements. This difference in rendered intensity is undesirable because it corresponds to the number of contemporaneously active print head elements, rather than to the intensities of the pixels in the source image being printed. The result is a printed image having undesired variations in intensity that do not accurately reflect the intensities of the pixels in the source image being printed. Examples of techniques for reducing the dependence of density on the number of contemporaneously active print head elements are disclosed in the above-reference patent entitled “Method and Apparatus for Voltage Correction.”
In conventional thermal imaging systems, printing multiple colors requires printing in multiple passes (one pass for each color). In the system disclosed in the above-referenced patent application entitled “Thermal Imaging System,” the print head is capable of writing up to three colors in a single pass on a single print medium. Each print line time is divided in up to three parts. It is possible to write one color in one part of the line time and another color in another part of the line time. The time division between the three colors, however, may not be equal. For example, if printing yellow and magenta, the yellow may be printed during a smaller fraction of the line time interval than magenta.
Integrating these and other features of a thermal printer into a single thermal imaging system presents a variety of challenges. For example, print data must be processed sufficiently quickly to provide the thermal print head(s) with a continual stream of data to avoid pauses in printing. Data must be stored and transmitted among components of the system efficiently to limit the size and cost of the overall system. Typically, the resulting integrated system includes a combination of analog and digital circuitry that is customized for use with a particular thermal printer. As a result, the system must typically be redesigned to work with a different thermal printer. Such redesign is tedious, time-consuming, and expensive.
What is needed, therefore, are improved techniques for processing print data and controlling print heads in a thermal printer.